Presidential candidates make the most of celebrity supporters, showing
them off in television ads and propping them on podiums to stand and
wave. No doubt Mike Huckabee’s aborted campaign for the Republican nomination got some sort of bump from those commercials of him with Chuck Norris, right?

Or maybe not. Politicians and pundits routinely claim that celebrity
endorsements have little sway on voters, and two economists set out
recently to test the premise. What they found was that at least one
celebrity does hold influence in the voting booth: Oprah Winfrey.

The economists, Craig Garthwaite and Timothy Moore of the University of Maryland, College Park, contend that Ms. Winfrey’s endorsement of Barack Obama last year gave
him a boost of about one million votes in the primaries and caucuses.
Their conclusions were based partly on a county-by-county analysis of
subscriptions to O: The Oprah Magazine and sales figures for books that
were included in her book club.

Those data points were cross-referenced with the votes cast for Mr.
Obama in various polling precincts. The results showed a correlation
between magazine sales and the vote share obtained by Mr. Obama, and
extrapolated an effect of 1,015,559 votes.

“We think people take political information from all sorts of sources
in their daily life,” Mr. Moore said in an e-mail message, “and for
some people Oprah is clearly one of them.”

In their as-yet-unpublished research paper on the topic, the economists
trace celebrity endorsements back to the 1920 campaign of Warren
Harding (who had Al Jolson, Lillian Russell and Douglas Fairbanks in his corner), and call Ms. Winfrey “a celebrity of nearly unparalleled influence.”

The economists did not, however, look at how Ms. Winfrey’s endorsement
of Mr. Obama may have affected her own popularity. A number of people —
women in particular — were angry that Ms. Winfrey threw her first-ever
political endorsement to a man rather than his female opponent.

The research did not try to measure the influence of other stars’
endorsements; for instance, no similar measures were available for
Obama supporters like the actress Jessica Alba or Pete Wentz of the
band Fall Out Boy. “If a celebrity endorsement is ever going to have an
empirically identifiable audience, then it is likely to be hers,” the
researchers said of Ms. Winfrey. Sorry, Chuck Norris.